As for English, I believe that the best monolingual dictionaries for advanced learners are:

1) Longman Dictionary of Contemporary English:

A key feature of it is its utilization of a limited amount of vocabulary to define words. Around 2000 words are used to write all of the definitions in the dictionary. Most of these words were developed from the General Service List of Michael West.

It also has around 170,000 corpus-based example sentences and 65,000 collocations.

2) Collins COBUILD Advanced Learner's Dictionary:

It is one of the first corpus-based dictionaries in English. The most interesting thing about it is its definitions. They are full sentences, no phrases. For example, the definition of the word "fruition" is the following:

"If something comes to fruition, it starts to succeed and produce the results that were intended or hoped for."

Because this definition is a full sentence, it gives the learner a lot of information. It shows that fruition is usually used in the phrase "come to fruition" (because the editors used a corpus). So now the learner can easily build his own CORRECT sentences with the word (e.g. "His hopes finally came to fruition" or "Will my plan ever come to fruition?"

In comparison with "traditional" dictionaries, we see that they don't have full-sentence definitions. For example:

"Fruition = the realization of something that was desired or hoped for."

After reading this definition, the learner might use the word in incorrect ways. He might think it is correct to say "What about your fruition?" or "Is this book your greatest fruition?". But both sentences are bad English!

Another example from COBUILD, is the definition of the verb "wag":

"When a dog wags its tail, it repeatedly waves its tail from side to side."

Traditional dictionary: "wag = to shake up and down or move from side to side."

COBUILD's definitions are very natural. They are sentences that could be said by English teachers or any native speaker of English. Having this dictionary feels like having a native speaker friend to answer the learner's questions about English.

I chose the Larousse French English dictionary to have at home for Creole/French speaking friend, because it explained the full meaning of "Metropolitan" and that was a word that online resources did not do a good job with at all and we had previously struggled with. I am hoping if the dictionary is helpful with that word it will be as helpful with others. It was the only dictionary and Barnes and Nobles that included the definition of "motherland" as well as "city".

I prefer Longman Dictionary. I used to have a very good Norwegian dictionary: Norsk-Engelsk Blå Ordbok. It is not huge, but I think that's enough for Trinn 3 learner for example. I used to have - because my friend borrowed it and still keeps it

Question to SmartRat: is the use of the word "friend" in this context still warranted?

Apart from that: a good example of a good dictionary would be The New College Latin and English Dictionary by John C.Traupman. Why? Well, simply because I normally find the words and phrases I look up in the English --> Latin section, even if they represent modern ideas and inventions. Some competitors (like the Neues Latein Lexikon from Lempertz) also have many of these words and expressions but try at all costs to find a classical or medieval example- and if that's impossible then they propose a clumsy circumlocution which no true Roman or medieval monk ever would have accepted.

Even worse: I once bought a red Gyldendal Danish --> Latin dictionary, and I suppose it is the only case where I have thrown an expensive dictionary in the dustbin in disgust. The reason: no matter what I tried to look up, chances were that it wasn't there because the dictionary apparently had been constructed as a simple inversion of a Latin --> Danish dictionary (even worse: it was a partial and not terribly succesful update of an even older dictionary built along this totally outdated principle).

The crux of the matter is that if you make a dictionary from language A to language B, then it MUST contain all the important words of language A as headwords, whether or not there exists a simple and idiomatic translation into language B. And then it is up to the compiling team to find the best possible translation under the circumstances - and to state the facts when they can't find anything suitable, instead of just being silent.

In hard copy, I've yet to see anything quite comparable to Longman's Contemporary or Collins COBUILD for any of my target languages, although I have a couple of dictionaries on my shelf that are a bit like Collins COBUILD, in addition to having used some fairly good ones for advanced learners on-line.

For German, I recommend the following hard copies for advanced learners:

The Langenscheidt monolingual dictionary has about 90,000 headwords, usages and example sentences. In addition to providing grammatical hints (i.e. gender, and endings of nom. plur. and gen. sing. for nouns, case governance for prepositions, 3rd person sing. form in present and/or preterite, auxiliary verb for perfect, and past participle for verbs) beside every headword, everything is defined in German with a few collocations usually shown providing hints about idiomatic usage (cf. hashimi's comment about fruition and how a simple definition may not necessarily guide the learner to use it idiomatically). All of the above is often enhanced by most headwords having at least one example sentence.

For those who don't know German, here's what I get:- the gender is neuter (i.e. das)- the genitive singular ending is -es (i.e. <-es>)- the main definition in 1 is "flesh of a dead animal" with an example sentence "Vultures and hyenas eat carrion."- I also get a compound meaning "scavenger" (i.e. Aasfresser - "carrion-eater")- a secondary definition used derogatively in speech to refer to a very unpleasant and mean-spirited person- a teritary definition found in coarse speech when preceded by kein "not (a, any)" means something like "nobody" clarified by the example sentence "That doesn't (even) interest a carcass!" (idiomatically: "Nobody gives a damn!")

Here's what I get:- the stress falls on the first syllable- it's intransitive- the 3rd person singular in preterite is ebbte ab while for perfect it conjugates sein "to be" for its auxiliary verb and its past participle (Partizip II) is abgeebbt- the main definition as something that becomes smaller, less numerous or weaker- collocations include using it with das Hochwasser "flood", das Interesse "interest" and der Lärm "(annoying/disruptive) noise"

The Stilwörterbuch by Duden is a little similar to the Großwörterbuch by Langenscheidt but focuses on collocations.

Here's what I get:- the gender is neuter (i.e. das)- the main definition in 1 is "carcass" with common collocations including "rotting, stinking carcass" and "to pick up the scent of, feed on a carcass" with an example sentence "Hyenas live off carrion."- there's also a fixed phrase or saying using Aas translated as "Where there's a carcass, vultures gather."- a secondary definition in 2 is used derogatively (and often as an insult) in colloquial settings to refer to a vile person with collocations linked to "mean" and "rotten, lazy " and example phrases "Such a refined đïçкћéàđ/carcass!" and "These immoral đïçкћéàđš/carcasses"- furthermore, using the term this way could betray a certain grudging respect on the speaker's part as demonstrable in the exclamation "Such a sly S.O.B./carcass!"- a less insulting but still colloquial use of the term along the lines of the secondary definition above is to use it to mean "nobody" as demonstrated in the example sentences "This place is known by nobody" and "There isn't even a carcass here/there (idiomatically: "Nobody's even there.")

Here's what I get:- several collocations with the verb: "The thrill / euphoria / dispute / turmoil slowly faded; the noise has faded; the interest in this subject among the public has meanwhile died down"

Although I think that the price of the latest edition of each dictionary is reasonable (around 25 Euros), you can sometimes find used copies of older editions (and not necessarily ones dating from before the spelling reforms of 1996) on Amazon Marketplace for a lot less.

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There are some on-line monolingual dictionaries for free that resemble in varying degrees the preceding two hard copies for German in providing definitions, example sentences, collocations and grammatical hints.

Similarly useful for advanced learners of certain big-name languages are on-line bilingual dictionaries hosted by PONS although these provide counterparts of the target language with those in the intermediary language rather than definitions in the target language. For some reason only the English <> German dictionary complements entries with example sentences drawn from the internet. Needless to say the massive unabridged bilingual dictionaries published by Collins or Oxford for FIGS are pretty good too, although for reasons of space usually have fewer example sentences and collocations, and resort more to abbreviations and cross-referencing than their online counterparts.

Great idea for a thread, will be interested to see what people recommend.

I agree that the Longman Dictionary of Contemporary English is excellent. And it's online.

The Oxford Learner's Dictionary is similar, also quite good. (For some reason, I tend to go for the LDOCE first, but I couldn't say why.)

I hadn't heard of the Collins COBUILD, but I think that's a great idea to give full-sentence definitions that really illustrate the context in which the word is used. Will start using that one too.

Finally, Longman also has the Longman Language Activator (I believe only in hard copy, not online). The way it works is, you look up a word, and you're directed to a group of words that express roughly the same idea, with an explanation of the subtle differences between them. For example, looking at the last sample page here, if you looked up either "dip" or "dunk", you would wind up at the section for "to put something in liquid for a short time". And it explains that for putting bread in soup or a biscuit/cookie in tea, we'd be more likely to use "dunk", whereas "dip" could be used more generally, such as when dipping your toes in water.

I would really love to find something that works the same way for languages other than English if anyone knows of one, especially for Spanish or French.